1. Field of the Invention
The present invention relates to a washing machine, and more particularly, to a direct drive motor for a washing machine and method of manufacturing the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing a configuration of a rotor of the direct drive motor.
2. Discussion of the Related Art
Generally, a pulsator type washing machine is a product that removes various dirt from clothes, linen and the like using detergent emulsification, a frictional power of water current, an impact power of a pulsator, etc. In particular, the pulsator type washing machine performs a washing process in a manner of automatically setting the washing method by detecting a quantity and type of a laundry using a sensor, supplying water to a level suitable for the detected quantity and type of the laundry, and then executes the washing process under a control of a microcomputer.
In a drum type washing machine, while a detergent, water and laundry are provided within a drum, a driving force of a motor is transferred to the drum to rotate. So, a washing is performed using a frictional force between the rotated drum and the laundry. Hence, the drum type washing machine is advantageous in preventing the laundry from being damaged and raveled with each other. And, the drum type washing machine has a washing effect of beating and rubbing the laundry.
Drum type washing machines according to a related art are classified into an indirect drive type washing machine of which drum is rotated by a motor indirectly transferring its driving force to the drum via a belt that connects a motor pulley and a drum pulley together and a direct drive type washing machine of which drum is directly connected to a rotor of a motor to be rotated.
In this case, the indirect drive type washing machine, in which the driving force of the motor is transferred via the belt connecting the motor and drum pulleys together, considerable noise and energy loss takes place in the course of transferring the driving force.
In order to problems of the related art drum type washing machine, a motor is assembled to a rear wall of a tub to directly transfer its driving force to a drum. So, the direct drive type drum type washing machine becomes more popular.
A configuration of a direct drive type drum type washing machine according to a related art is explained as follows.
FIG. 1 is a cross-sectional diagram of a direct drive type drum type washing machine according to a related art.
Referring to FIG. 1, a direct drive type drum type washing machine according to a related art consists of a tub 2 provided within a cabinet 1, a drum 3 provided within the tub 2, a washing shaft 4 connected to the drum 3 to transfer a driving force of a motor 5 to the drum 3, an a bearing (not shown in the drawing) provided to an outer circumference of each end of the washing shaft 4.
A door 21 is provided to a front side of the cabinet 1 and a gasket 22 is provided between the door 21 and the tub 2.
A hanging spring 23 is provided between an inside of a topside of the cabinet 1 and an upper outer circumference of the tub 2 to support the tub 2. And, a frictional damper 24 provided between an inside of a bottom side of the cabinet 1 and a lower outer circumference of the tub 2 to attenuate vibration of the tub 2 in executing a dewatering cycle.
In this case, the motor 5 consists of a stator 7 assembled to a rear wall portion 200 of the tub 2 and a rotor 6 configured to enclose the stator 7. A driving force of the rotor 6 is directly transferred to the drum 3 via the washing shaft 4. Namely, the drum 3 is driven according to a direct drive type without a power transmission via separate pulley or belt configuration.
A configuration of a direct drive type motor for a washing machine according to a related art is explained with reference to FIG. 2 and FIG. 3 as follows.
FIG. 2 is a perspective diagram of a stator of the motor shown in FIG. 1 and FIG. 3 is a perspective diagram of a rotor of the motor shown in FIG. 1.
Referring to FIG. 2 and FIG. 3, the motor 5 consists of the stator 7 and the rotor 6. If an electric power is applied to the motor 5, a rotational magnetic field is generated between a coil 8 wound on the stator 7 and a permanent magnet 16 to rotate the rotor 6.
In this case, the stator 7 consists of the coil 8, a stator core 10 provided with a plurality of teeth 9 having the coil 8 wound thereon and an insulator 12 insulating the coil 8 and the stator core 10 from each other. In particular, the stator 7 is fixed to the rear wall portion of the tub 2 via bolts or the like fitted into locking holes 11, respectively.
Alternatively, although FIG. 2 shows that the locking holes 11 are provided to the insulator 12 and the stator core 10, the locking holes 11 can be provided to the insulator 12 only in a manner of configuring the stator core 10 to have protrusions projected inward in a radial direction for the locking holes.
The rotor 6 is provided to enclose the stator 7 and is connected to the washing shaft 4 connected to the drum 3 by perforating the tub 2. Namely, the drum 3 is rotated by a rotation of the rotor 6 via the washing shaft 4.
And, a bearing housing (not shown in the drawings) is provided between a rear wall portion 200 of the tub 2 and the stator 7 to rotatably support the washing shaft 4.
Configurational details of the rotor 6 are explained as follows.
First of all, the rotor 6 consists of a rotor frame 15 and a permanent magnet 16. The rotor frame 15 consists of a sidewall part 13 and a backside wall part 14. A perforated hole is provided to a central portion of the backside wall part 14. And, a washing shaft passes through the perforated hole to directly drive a drum. Moreover, the permanent magnet 16 is provided in a circumferential direction within the sidewall part 13 of the rotor frame 15.
In this case, a magnetic path needs to be formed to enable a magnetic flux generated from the permanent magnet 16 to pass through. Yet, if the rotor frame 15 is formed of a magnetic substance, it is unnecessary to include a magnetic back yoke to form a magnetic path therein. Namely, the sidewall part 13 of the rotor frame 15 plays a role as a back yoke for forming the magnetic path, whereby the rotor frame 15 and the back yoke can be formed on one body.
Meanwhile, a connector 30 connecting the washing shaft 4 and the rotor frame 15 together is provided in one body of the rotor frame 15 to the perforated hole. Alternatively, the connector 30 is assembled to the rotor frame 15 using a separate locking means.
In this case, a serration 31 is provided to a central part of the connector 30 to transfer a driving force of the rotor 6 to the washing shaft 4 by the serration joint with the washing shaft 4.
A plurality of cooling fins 16, cooling holes 17 and drain holes 18 are provided in a circumferential direction to the backside wall part 14 of the rotor frame 15, thereby preventing the stator from being overheated and facilitating water to be drained.
A plurality of embossed parts 19 are provided to the backside wall part 14 of the rotor frame 15 to raise rigidity of the rotor frame 15, and a step sill 20 is provided to an inner side of the sidewall part 13 of the rotor frame 15 in a circumferential direction to support the permanent magnet 16.
FIG. 4 is a graph of a magnetization waveform of a permanent magnet of a direct drive type motor according to a related art.
In the related art, a permanent magnet is separated into a plurality of pieces to be assembled to an inner surface of a sidewall of a rotor in a circumferential direction. And, the permanent magnetic pieces are magnetized to enable N and S polarities to alternate along the circumferential direction.
So, the permanent magnet, as shown in FIG. 4, has a waveform close to a square wave according to an angle to generate a point at which the magnetic polarity abruptly changes according to the circumferential direction of the inner surface of the sidewall of the rotor.
In general, a related art washing machine motor is the BLDC (brushless DC motor) and a rotational speed of a rotor is controlled by an inverter. In particular, an AC voltage of a power is converted to a DC voltage, converted to a 3-phase (u,v,w) AC voltage, and then applied to the motor.
The voltage applied to the motor has a PWM (pulse width modulation) waveform, and an amplitude and frequency of the voltage applied to the motor are adjusted by controlling a size of a duty ratio.
Meanwhile, the inverter is driven according to a square wave drive or a sine wave drive, which is discriminated according to whether the drive has a conduction angle 180° or a 120° conduction angle in six type switching sequences of a 3-phase inverter.
Since a harmonic component is small in case of the 120° conduction angle, a waveform gets closer to the sine wave. So, the sine wave drive is more popular for a washing machine motor drive in general.
However, in a motor driven by the sine wave drive and having a permanent magnet magnetized in a square waveform, a cogging torque and the like are generated due to the square waveform magnetization of the motor. And, this problem causes a torque ripple that pulsates as well as a drive torque for driving the motor. So, motor efficiency is lowered but vibration and noise are raised.
In particular, the cogging torque is generated from an abrupt fluctuation of a mutual reaction between the permanent magnet 16 and a winding part having the coil wound thereon while the motor is driven. And, the cogging torque is raised if a magnetized pattern of the permanent magnet gets closer to the square wave.
So, in a direct drive motor of a general washing machine for a sine wave drive, many efforts have been made to solve the problem of the cogging torque raised due to the square wave magnetization of the permanent magnet and the corresponding vibration and noise problems.
Meanwhile, such a general permanent magnet as an AlNiCo magnet, a ferrite magnet and the like is normally formed by sintering or the like. So, the magnet is considerably heavy to cause an energy loss attributed to an inertial force and difficult to be assembled to a rotor frame.
Meanwhile, a number of permanent magnets need to be attached to the rotor frame in a circumferential direction. So, the magnet attaching process is complicated and a reliability of the attachment is degraded.